The present invention relates to a semi-transmission/reflection-type liquid crystal display device that displays an image by selectively using transmission light and reflection light.
Being thin, light, and low in power consumption, liquid crystal display devices in which a liquid crystal display panel is used as an image generating means are employed as display devices in a wide variety of electronic equipment, such as notebook-sized personal computers, word processors, electronic notes, cellular phones, and camera-incorporated video recorders.
In contrast to CRTs and plasma display panels, liquid crystal display panels display an image by controlling the quantity of light that has entered the panel from the outside instead of emitting light by itself. When equipped with color filters of plural colors as light control elements, liquid crystal display panels can display a color image of multiple colors.
Liquid crystal display devices that are used most commonly at present are transmission-type devices in which an illumination light source, called a backlight that uses a fluorescent tube, is provided on the back side of a liquid crystal panel, and an image is displayed by controlling the quantity of light (part of light emitted from the backlight) that passes through the liquid crystal panel.
However, in such transmission-type liquid crystal display devices, the power consumption of the backlight accounts for approximately one half of the total power consumption. This is a major factor in shortening the usable time in a case where portable electronic apparatuses as exemplified above are of a battery-driven type. Transmission-type liquid crystal display devices have another problem, in that, when they are used in a bright outdoor environment, for example, ambient light is reflected by the surface of the display area and a displayed image becomes hard to recognize.
Among liquid crystal display devices that are always used in a carried state in a bright environment such as found outdoors, there are reflection-type liquid crystal display devices that usually do not use a backlight, but are equipped with a reflection plate, and control the quantity of reflection light (part of ambient light) with the liquid crystal layer. An example of such liquid crystal display devices is one that performs both transmission-type display and reflection-type display using a semitransparent reflecting film (e.g., Japanese Unexamined Patent Publication No. Hei. 7-333598).
Another example of the above type of liquid crystal display device is one in which each pixel electrode is composed of two regions that comprise a reflection region and a transmission region (e.g., Japanese Unexamined Patent Publication No. Hei. 7-333598).
However, in the above conventional liquid crystal display devices, the display quality varies depending on the use environment (light-source-related environment). That is, a display that is performed by using reflection light (reflection light mode) and a display that is performed by using transmission light (transmission light mode) having different contrast ratios. Further, a coloration phenomenon may occur in black-and-white display and hue deviation may occur in color display. The difference in contrast ratio is caused by the fact that the black display luminance (off transmittance) and the white display luminance (on transmittance) are different between the case where reflection light is used and the case where transmission light is used. This phenomenon lowers the legibility of a displayed image. The hue deviation is a phenomenon that the hue shifts to the bluish side particularly in the case where transmission light is used. This deteriorates the color reproduction performance.